On a lot of inspections, I’ll find what looks like a decent patch job—until I poke around and there’s damp decking or even soft spots underneath. Ever notice how fast OSB swells when water sneaks in? I’m with you on checking the underlayment. Saw one last fall where the flashing looked fine, but the felt was basically compost. Curious if anyone’s had luck with some of those newer synthetic underlayments when redoing sections?
Curious if anyone’s had luck with some of those newer synthetic underlayments when redoing sections?
I’ve tried a couple of the synthetics—RhinoRoof and Deck-Armor, mostly—when patching up after flashing failures. I’ll say, they’re definitely tougher than old-school felt, especially if you’re working in humid spots or you know the repair might sit exposed for a bit. But I’m not totally sold on them for every job. The main thing is, they don’t absorb water like felt, so if you get a leak, the water tends to run right down to the next weak spot. That can be good or bad, depending on how well everything else is layered.
Here’s how I usually handle it when I find “decent” looking patches but soft decking underneath:
1. Pull back the shingles and flashing, even if it looks fine. If the underlayment is mushy or black, it’s gotta go.
2. Check the decking with a screwdriver or awl. OSB especially—like you said—swells up and loses strength fast. If it’s spongy, cut out and replace that section.
3. For underlayment, I’ll use synthetic if it’s a small area and I know I can tie it in well with the rest of the roof. If it’s a big patch or an older roof with felt everywhere else, sometimes I just stick with felt for consistency.
4. Flashing: If the aluminum’s pitted or bent, replace it. Don’t try to caulk over holes or cracks—it never lasts more than a season or two.
One thing I’ve noticed with synthetics: they’re slippery as heck when wet, so watch your footing. Also, some brands don’t seal around nails as well as felt does, so if you’re in a high-wind area or get wind-driven rain, that can matter.
Had one job last year where someone had patched with synthetic underlayment but left the old felt underneath. Water got between the layers and just sat there—decking rotted out anyway. Lesson learned: don’t sandwich materials unless you’re sure everything’s bone dry.
Long story short, synthetics are good tools but not magic. If the deck’s bad or the flashing’s shot, there’s no shortcut—gotta open it up and do it right. Patch jobs only work if everything underneath is solid. Otherwise, you’re just buying time... and not much of it.
I’m right there with you on not trusting a patch if the decking’s soft underneath. I had a spot over my porch where the flashing was corroded and someone before me just slapped new shingles over it—didn’t last a year before water started seeping in again. I tried one of those synthetic underlayments (I think it was Titanium UDL), and it held up fine for a few months, but honestly, it didn’t solve the real problem. Ended up pulling everything off, replacing the rotten OSB, and going back to felt for that section since the rest of the roof was still original. I do like how tough the synthetics are, but I agree, they’re not a cure-all. If the base isn’t solid, nothing on top will hold up for long.
If the base isn’t solid, nothing on top will hold up for long.
That’s been my experience too. I’ve seen plenty of “quick fixes” where folks just patch over bad decking or corroded flashing, and it always comes back to bite you. I had a rental where someone tried to seal over failing aluminum flashing with some kind of roof cement—looked fine for a season, but by the next spring, water had tracked under the shingles and rotted out a whole section of sheathing. Synthetic underlayment is tough, but it won’t compensate for structural issues underneath. Sometimes you just have to bite the bullet and do a full tear-off, even if it’s a pain.
Sometimes you just have to bite the bullet and do a full tear-off, even if it’s a pain.
I had a similar situation a few years back—tried to save some time by just patching over the old flashing on a porch roof. Looked fine until a big rainstorm, then water started showing up inside. Ended up costing more to fix the rot than if I’d just replaced the flashing from the start. Lesson learned: if the flashing’s shot, covering it up is just delaying the inevitable.
